1. Field of the Invention
The present invention relates to silver (Ag) alloy reflective films for optical information recording media, Ag alloy sputtering targets for the deposition thereof, and optical information recording media. More specifically, it relates to reflective films which have low thermal conductivities, low melting temperatures, high reflectivities, and high corrosion resistance and therefore enable marking of optical information recording media such as CDs, DVDs, Blu-ray Discs, and HD-DVDs typically using laser after the preparation of the media. The present invention also relates to sputtering targets for the deposition of the reflective films, and optical information recording media having the reflective films.
2. Description of the Related Art
Optical information recording media (optical discs) include various types, and the three main types categorized by the writing/reading system are read-only, write-once, and rewritable optical discs.
Of these optical discs, read-only discs have recorded data formed by concave and convex pits on a transparent plastic substrate upon the manufacture of the discs, and a reflective film layer mainly containing, for example, Al, Ag, or Au is arranged on the recorded data. The data are read out (reproduced) by detecting phase difference or reflection difference of laser beam applied to the discs. Certain optical discs include a substrate containing recording pits and a reflective film layer arranged on the recording pits, and another substrate bearing recording pits and a semi-reflective layer arranged on the recording pits. The two substrates are laminated, and the data recorded on the two layers are read out. Data recorded on one side according to this recording/reproducing system are read-only data that cannot be additionally wrote and altered, and optical discs using this system include CD-ROMs and DVD-ROMs. FIG. 1 is a schematic view of a sectional structure of a read-only optical disc. The optical disc in FIG. 1 includes a polycarbonate substrate 1, a semi-reflective layer (Au, Ag alloy, and Si) 2, an adhesive layer 3, a total-reflective film layer (Ag alloy) 4, and an ultraviolet-curable resin protective layer 5.
These read-only optical discs are produced in quantities, and information is recorded upon production of the discs by pressing with stampers having patterns of the information. Thus, IDs cannot be significantly given to individual discs. However, read-only optical discs individually having IDs formed using a dedicated system such as a label gate system or a burst cutting area (BCA) system after the preparation of the discs are being standardized typically for preventing unauthorized copying, improving traceability in distribution of products, and increasing added values of products. The ID marking (recording) is mainly carried out by a method of applying laser beam to discs after production to melt an Al alloy in the reflective film and to form holes therein.
Aluminum alloys, such as Al—Mg alloys according to Japanese Industrial Standards (JIS) 6061, are distributed in quantities as general construction materials, are inexpensive and are thereby widely used as reflective films of read-only optical discs.
The JIS 6061 Al alloys; however, are not intended to undergo laser marking and thereby have following problems.
Specifically, the Al alloys have high thermal conductivities and require high laser power for marking, which results in damages on base materials including polycarbonate substrates and adhesive layers. Additionally, voids formed as a result of laser marking invite corrosion of the reflective film in constant temperature and constant humidity tests conducted after laser marking, because the Al alloys have low corrosion resistance.
Recordable optical discs often use Ag alloys having higher reflectivities as reflective films. The resulting reflective films containing Ag alloys, however, show decreased reflectivities at high temperatures due to cohesion of Ag reflective films caused by the low thermostability of Ag. Various proposals have been made to solve this problem and to improve the durability. For example, Japanese Laid-open (Unexamined) Patent Application Publication (JP-A) No. 2002-15464 discloses a technique of inhibiting the grain growth (cohesion) of Ag by incorporating 0.1 to 3 atomic percent of a rare-earth element into Ag. JP-A No. 2004-139712 discloses a technique of further improving reflectivity and durability while ensuring high thermal conductivity by incorporating Bi or Sb into Ag.
JP-A NO. 04-252440 discloses a method for reducing the thermal conductivity of an Ag alloy by incorporating Ge, Si, Sn, Pb, Ga, In, Tl, Sb, or Bi into Ag. JP-A No. 04-28032 discloses a method for reducing the thermal conductivity of an Ag alloy by incorporating Cr, Ti, Si, Ta, Nb, Pt, Ir, Fe, Re, Sb, Zr, Sn, or Ni into Ag. The resulting reflective films obtained according to these techniques, however, are not intended to be melted and removed by laser irradiation, and some of them show increasing melting temperatures with decreasing thermal conductivities thereof. Silver alloys (Ag alloys) satisfying requirements as Ag alloys for laser marking have not yet been provided.
As is described above, Ag alloys provided for laser marking must have low thermal conductivities, low melting temperatures, high corrosion resistance, and high thermostability.
Current reflective films for read-only optical discs use JIS 6061 series Ag alloys, but these Ag alloys do not satisfy the requirements for laser marking in thermal conductivity and corrosion resistance.